POMs are a unique class of inorganic metal-oxygen clusters. They consist of a polyhedral cage structure or framework bearing a negative charge which is balanced by cations that are usually external to the cage, and may also contain centrally located heteroatom(s) surrounded by the cage framework. Generally, suitable heteroatoms include Group 13 to 16 elements such as phosphorus, antimony, silicon and boron. The framework of POMs comprises a plurality of metal atoms (addenda), which can be the same or different, bonded to oxygen atoms. Up to now the framework metal is substantially limited to a few elements including transition metals from Group 5 and Group 6 in their high oxidation states, e.g., tungsten (VI), molybdenum (VI), vanadium (V), niobium (V) and tantalum (V).
The first example in the POM family is the so-called Keggin anion [XM12O40]3− with X being a heteroatom selected from a wide variety of elements such as phosphorus, and M being a Group 5 or Group 6 metal such as Mo or W. These anions consist of an assembly of corner- and edge-shared MO6 octahedra of the metals of Groups 5 or 6 around a central XO4 tetrahedron.
In the past, there have been increasing efforts towards the decoration of polyoxoanions with various organic and/or transition metal complex moieties with the aim of generating new catalyst systems as well as functional materials with interesting optical, electronic and magnetic properties. In particular, transition metal-substituted polyoxometalates (TMSPs) have attracted continuously growing attention as they can be rationally modified on the molecular level including size, shape, charge density, acidity, redox states, stability, solubility, etc. In particular, POMs comprising transition metals of Groups 9 to 11 of the periodic table of the elements (e.g., Pd and Pt) are of interest because they are thermally and oxidatively stable and possess highly attractive catalytic properties.
For example, Angus-Dunne et al. describe the preparation of palladium-substituted sandwich-type POM K2Na6[Pd2W10O36].22H2O (see: J. Chem. Soc., Chem. Commun. 1994, pp. 523-524). This polyanion is composed of two W5O186− moieties linked by two palladium(II) ions in square-planar environments.
Lee et al. disclose the structure of several platinum(IV)-substituted POMs, such as (CH6N3)8[α-SiPt2W10O40].6H2O, a Keggin-type polyanion in which two addenda atoms are replaced by Pt atoms (Acta Crystallographica, Section C, 2003, C59, m152-m155), as well as K2[H6-α-PtMo6O24]5H2O (Acta Crystallographica, Section C, 1994, C50, pp. 1657-1659), (CH6N3)8[PtW6O24] (Acta Crystallographica, Section E, 2003, E59, m116-m118), (NH4)4.5[H3.5-α-PtMo6O24].1.5H2O, (NH4)4[H4-β-PtMo6O24].1.5H2O and K3.5[H4.5-α-PtMo6O24].3H2O (Bulletin of the Korean Chemical Society 1994, 15, pp. 37-45), which all have the so-called Anderson-Evans structure.
Kortz et al. report on the palladium(II)-substituted, dimeric, lone pair containing polyanion [Cs2Na(H2O)10Pd3(α-SbW9O33)2]9− (Inorg. Chem. 2004, 43, pp. 3915-3920). This polyanion was synthesized by reacting Pd(CH3COO)2 with [α-SbW9O33]9− in aqueous acidic medium. The square-planar palladium(II) ions are located in the central belt of the sandwich-type structure connecting two (α-SbW9O33) Keggin moieties via bonding to oxygen atoms of the WO6 octahedra.
Hill et al., Science 2004, 306, pp. 2074-2077, disclose the synthesis of the Pt (IV)-containing compound K7Na9[Pt(O)(H2O)(PW9O34)2].21.5H2O. The polyanion [Pt(O)(H2O)(PW9O34)2]16− is composed of two (A-α-PW9O34) Keggin units linked by an octahedral platinum(IV) center with terminal oxo and water ligands. Attempts to reproduce this complex proved unsuccessful.
Kortz et al., Inorg. Chem. 2004, 43, pp. 8367-8372, describe the synthesis of a palladium(II)-substituted tungstosilicate, [Cs2K(H2O)7Pd2WO(H2O)(A-α-SiW9O34)2]9−. This polyanion was synthesized by reacting Pd(CH3COO)2 with [A-α-SiW9O34]10− in aqueous acidic medium. It is composed of two (A-α-SiW9O34) Keggin moieties which are linked via a central belt consisting of a {WO(H2O)}4+ group, two Pd(II) centers, one potassium ion and two cesium ions. The palladium(II) ions exhibit square-planar coordination geometry as they are only ligated to four oxo groups of the polyanion backbone. The potassium and cesium ions are also coordinated by terminal water molecules.
Moreover, Kortz et al. report on the palladium(II)-substituted, lone pair containing polyanion [Cs2Na(H2O)8Pd3(α-AsW9O33)2]9− (Eur. J. Inorg. Chem. 2005, pp. 3034-3041). This polyanion was synthesized by reacting PdCl2 with [α-AsW9O33]9− in aqueous acidic medium. The square-planar palladium(II) ions are located in the central belt of the sandwich-type structure connecting two (α-AsW9O33) Keggin moieties via bonding to oxygen atoms of the WO6 octahedra. This polyanion can be considered as the As-analogue of the above mentioned [Cs2Na(H2O)10Pd3(α-SbW9O33)2]9−.
Kortz et al. also report on the palladium(II)-substituted, lone pair containing polyanion [Na2(H2O)2PdWO(H2O)(α-AsW9O33)2]10− (Eur. J. Inorg. Chem. 2005, pp. 3034-3041). This polyanion was synthesized by reacting PdCl2 with [AS2W19O67(H2O)]14− in aqueous acidic medium. It is composed of two (α-AsW9O33) Keggin moieties which are linked via a central belt consisting of a {WO(H2O)}4+ group, a square-planar Pd(II) center and two sodium ions. One of the two sodium ions in the central belt is located in an addenda atom position, which appears to decrease the stability of this polyanion in solution.
Recently, Hill et al. describe the preparation of a Pd(IV)-oxo compound, K10Na3[PdIV(O)(OH)WO(OH2)(PW9O34)2], by reacting PdSO4 with [A-α-PW9O34]9− yielding [PdII3(PW9O34)2]12− which undergoes rapid loss of Pd(II) to form [PDIIWO(OH2)(PW9O34)2]12− and oxidation of this compound resulted in the Pd(IV)-oxo complex (Hill et al., J. Am. Chem. Soc. 2005, 127, pp. 11948-11949). This polyanion is composed of two (A-α-PW9O34) Keggin moieties which are linked via a central belt consisting of a {WO(H2O)}4+ group and a {PdO(OH)}+ group. The tungsten center in the belt has an internal oxo ligand and an external water ligand, whereas the Pd center has an internal oxo and an external hydroxy ligand.
Recently, Kortz et al. report on the Pt(IV)-containing polyvanadate [H2PtIVV9O28]5− which is prepared by a one-pot reaction of Na2[Pt(OH)6] with NaVO3 (Kortz et al., Angew. Chem. Int. Ed. 2008, 47, pp. 793-796). The obtained polyanion has a decavanadate structure in which one of the two central addenda sites is occupied by platinum(IV).
However, there is still a need for late transition metal-containing POMs showing useful properties in homogeneous or heterogeneous catalytic applications.
Therefore, it is an object of the present invention to provide Group 9 to 11 transition metal-containing POMs which are useful as catalysts in homogeneous and heterogeneous oxidation reactions of organic substrates. Furthermore, such Group 9 to 11 transition metal-containing POMs should be easy and reproducible to prepare. Moreover, they should be useful as precursors for preparing mixed metal oxide catalysts and finely distributed metal clusters in a highly reproducible manner.